<!DOCTYPE html>
<html>
<head>
  <style type="text/css" media="screen">
    .wrapper {
      outline: 1px solid silver;
      width: 280px;
      height: 280px;
      -webkit-box-sizing: border-box;
      padding: 90px;
      margin: 4px;
      display: inline-block;
    }
    
    .box {
      position: relative;
      width: 100px;
      height: 100px;
      display: inline-block;
    }
        
    .indicator {
      position: absolute;
      outline: 0 solid rgba(0, 255, 0, 0.05);
    }
    
  </style>
</head>
<body>
  
  <!-- The extent of the shadow should approximately match the pale green boxes. -->
  <div class="wrapper">
    <div class="indicator box" style="outline-width: 10px"></div>
    <div class="box" style="box-shadow: 0 0 10px black"></div>
  </div>

  <div class="wrapper">
    <div class="indicator box" style="outline-width: 20px"></div>
    <div class="box" style="box-shadow: 0 0 20px black"></div>
  </div>
  <br>

  <div class="wrapper">
    <div class="indicator box" style="outline-width: 50px"></div>
    <div class="box" style="box-shadow: 0 0 50px black"></div>
  </div>

  <div class="wrapper">
    <div class="indicator box" style="outline-width: 90px"></div>
    <div class="box" style="box-shadow: 0 0 90px black"></div>
  </div>

</body>
</html>
